There are already known various methods of and apparatus constructions for removing carbon monoxide from gaseous media, among them such capable of removing carbon monoxide by oxidation in the presence of gaseous oxygen from gaseous media that additionally contain other oxidizable or otherwise reactable substances, such as hydrogen, using catalysts that selectively or preferentially promote the desired conversion of carbon monoxide to carbon dioxide.
One such approach is disclosed in an article authored by M. L. Brown et al titled "Purifying Hydrogen by . . . Selective Oxidation of Carbon Monoxide", appearing in Volume 50, No. 10 of Industrial and Engineering Chemistry, pp. 841-844 (1960). As discussed there in the context of ammonia synthesis, certain catalysts, such as alumina-supported platinum, are capable of selectively promoting the oxidizing reaction of carbon monoxide to carbon dioxide with only small or negligible losses of hydrogen contained in the gaseous medium being treated to water formation, which is not bothersome and may even be desirable in this context, so long as the temperature at which the reaction takes place is above a threshold temperature. However, it is also disclosed in this article that there is a relatively high limiting value below which the carbon monoxide concentration cannot be reduced when the oxidation reaction is conducted at such relatively high temperatures in a single stage. This limitation is primarily if not exclusively attributable to a reverse shift reaction in which carbon dioxide reacts with hydrogen to form carbon monoxide and water. On the other hand, this article also mentions that, when it was attempted to conduct the selective oxidation reaction on the incoming gaseous medium, which has a relatively high carbon monoxide concentration, at temperatures below the threshold temperature, the catalyst very rapidly became ineffective and did not become effective again merely as a result of a subsequent raising of the reaction temperature to above the threshold temperature; rather, it was necessary first to purge the catalyst using a gaseous medium substantially devoid of carbon monoxide at temperatures above the threshold temperature, followed by conducting the selective oxidation reaction at such elevated temperatures. Thus, this article indicates that it is impossible or at least not feasible to treat the incoming gaseous medium at temperatures below the threshold temperature.
However, even in the above context, it is often desirable to reduce the carbon monoxide concentration to below the limiting value before the gaseous medium is supplied to the ammonia synthesis equipment proper. To this end, the above article proposes to use two consecutive selective carbon monoxide oxidation stages, with a carbon dioxide removal apparatus being interposed between such consecutive oxidation stages. Even here, however, the selective oxidation reaction is conducted at above the threshold temperature in both of the oxidation stages; yet, because of the removal of the carbon dioxide from the gaseous medium between the two oxidation stages and the resulting dearth of carbon dioxide that could participate in the aforementioned reverse shift reaction from the gaseous medium entering the second oxidation stage, the severity of such reaction in the second oxidation stage is drastically reduced, resulting in a carbon monoxide concentration in the gaseous medium leaving the second oxidation stage that is considerably below that achievable in the first oxidation stage or, for that matter, in a single-stage oxidation device.
Even the latter approach, however, leaves much to be desired. For one, the need for providing the carbon removal device between the two selective oxidation stages not only increases the complexity of the gaseous medium treatment equipment, but also significantly adds to its cost. Moreover, and possibly more importantly, there is still a limit, albeit lower than in the single-stage approach, below which the carbon monoxide concentration cannot be reduced when using the two-stage approach. Yet, there are applications, such as in treating a gaseous fuel to be supplied to a fuel cell, where even such a reduced carbon monoxide concentration is undesirably high.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a method of selectively removing carbon monoxide from a gaseous medium by oxidation, which method does not possess the disadvantages of the known methods of this kind.
Still another object of the present invention is so to develop the selective oxidation method of the type here under consideration as to achieve reduction of the carbon monoxide concentration in the gaseous medium to a minimum.
It is yet another object of the present invention to devise a selective carbon monoxide removal apparatus particularly suited for performing the method of the above type.
A concomitant object of the present invention is to design the apparatus of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.
A yet further object of the present invention is to present a method and apparatus of the above type which avoid the otherwise existing need for removing carbon monoxide from the gaseous medium in order to be able to reduce the carbon monoxide concentration to below a level achievable in the absence of such removal.